location importer tweaks

- disable upload button after click
- friendlier validation UI
- fix typo
don't force subclasses to override function if not necessary
remove unnecessary page_context overrides
remove future url import
Merge pull request #2763 from dimagi/location-upload-tweaks

location importer tweaks
Merge pull request #2765 from dimagi/url-future

remove future url import
Merge pull request #2764 from dimagi/cz

don't force subclasses to override function if not necessary
Merge branch 'master' into product-upload